In digital systems, a common problem is the distribution of logic signals over relatively long distances. Electrically, this distribution of logic signals requires that a logic circuit be able to produce a signal having sharp rise and fall times with minimum transmission delay when working into a highly capacitive load. Thus, the requirement is that a transmitting logic circuit, commonly called a line driver, have a very low output impedance which will allow the rapid transfer of current to or from the capacitive load and thus minimize the rise and fall time of the transmitted logic signal. A line driver which has the capability of delivering current to the capacitive load and from the capacitive load so that both the rise time and the fall time of a propagated logic signal are controlled by the line driver's output impedance characteristic is called bidirectional since it drives current in both directions.
Several approaches have been taken in the various electrical circuits used to accomplish a bidirectional line driver function. One widely used approach is to use NPN transistors and PNP transistors in a tandem connection driving a common terminal. This type configuration has the advantage that the NPN transistor drives current in one direction and the PNP transistor drives current in the other direction to attain a bidirectional drive capability with relatively simple input circuitry. The major disadvantage with this approach is the fact that two different transistor types are required. In integrated circuit embodiments, it is very difficult to obtain NPN transistors and PNP transistors having optimized operating characteristics such as current gain at high frequencies within a single integrated circuit chip. Typically, in integrated circuits, NPN transistors with good high frequency performance are the easiest to fabricate and PNP transistors with reasonable high frequency performance can only be fabricated on the same integrated circuit chip by sacrificing the high frequency performance of the NPN transistors. Thus, a bidirectional line driver which can be implemented using only transistors of a single type, particularly NPN transistors, offers significant performance advantages.